Yes, this is how a general economist analyses social situations:
An invitation is an option that can be exercised at any time before the date of the party. The people who did not respond immediately are waiting to decide whether to exercise the option. If she’s a true friend then this is because she has a potential conflict that would prevent her attending. … When she is sure there is no conflict she will say yes.
The other people are hoping for an excuse not to come. Once they get a better offer, manage to schedule a conflicting business trip, or otherwise commit themselves, they will send their regrets.
Those who want to come but haven’t gotten out of their conflict give up and send their regrets. Those who hoped to get out of it but failed to come up with a believable excuse give up and accept. … So, a simple measure of how much your friends like you is the proportion of acceptances that arrive in the final days.
It isn’t that economists are ignorant of social realities – we might just spend a little too much time analysing what is going on, a reality that is obvious when you see us uncomfortably twitching around 😉
I would note that, in the case of Facebook, the situation is a little different now. Previously you would see invites on the home page – so you had to make a conscious decision everytime you logged in (so about 24 times a day). However, now you receive an email and don’t hear about it again until close to the date – as I get so much spam I don’t often notice events until close to the date.
So I’m sure I’ve got a valid excuse for not accepting until the last minute 😉
Update: Eric Crampton has a great example of this.